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Single p 0 photoproduction at SPring-8/LEPS

Single p 0 photoproduction at SPring-8/LEPS. Mizuki Sumihama Osaka university, RCNP. JPS meeting March 2007. Introduction.

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Single p 0 photoproduction at SPring-8/LEPS

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  1. Single p0 photoproduction at SPring-8/LEPS Mizuki Sumihama Osaka university, RCNP JPS meeting March 2007

  2. Introduction Pion photoproduction is well studied experimentally and theoretically as a spectroscopy of N* and D* resonances. The proprieties of many resonances are determined at W < 1.7 GeV. However above the resonance region, the production mechanism is not well studied partly due to lack of data. We measured single p0 photoproduction in 1.9-2.3 GeV in total energy at backward angles where less experimental data. The production mechanism will be investigated in a transition range from nucleon-meson degrees of freedom to quark-gluon degrees of freedom. High mass resonances, u-channel contribution, quark counting rule…

  3. Diagram in tree level • W < 1.7 GeV • Models are well established. • Data are well explained by • four-star N* and D*. • W ~1.9 GeV • There are one or two star • N* and D* but doubtful. • W > 1.9 GeV • at very backward angles • non-resonant u-channel • is expected to be dominant. • Much higher, W > 3 GeV • in 60s, 70s. Regge baryon • pole explains data well. • Regge ~s2a(u)-2 at u~small, • follow quark counting rule.(JLab) t-channel (forward) s-channel g p0 g g p0 p0 p,r,w p p p p p p 1.9 – 2.3 GeV +Backward - medium p p g u-channel s-channel p0 p p N*, D* Born term List of four-star nucleon resonances P33(1232),P11(1440), D13(1520),S11(1535), S31(1620), S11(1650), D15(1675), F15(1680), D33(1700), P13(1720), F35(1905),P31(1910) and F37(1950) in MAID2005.

  4. Measurement of g + p  p + p0 Linearly polarized photons p Detect protons By spectrometer g Liquid hydrogen p p0 in missing mass • Detect protons by LEPS spectrometer at forward angles • Identify p0 in missing mass spectrum. • Measure differential cross sections and photon beam • asymmetries at backward angles -1 < cosqcm < -0.6. • Polarization degree is ~95% at the maximum Eg, 2.4 GeV.

  5. LEPS spectrometer – forward acceptance g +-~10o in y +-~20o in x TOF wall Aerogel Cherenkov (n=1.03) Dipole Magnet (0.7 T) Start counter Liquid Hydrogen Target (50 mm thick) Linearly polarized MWDC 3 Silicon Vertex Detector 1m MWDC 1 MWDC 2

  6. Particle identificationby time-of-flight and momentum measurements • Proton selection with 4s. • Kaon/pion contamination is negligible.

  7. Missing mass spectrum gppX 1/10 total statistics LH2 target ~18000 counts Z-vertex distribution Measurement region: 1.5 < Eg< 2.4 GeV 1.9 < W < 2.3 GeV -1 < cosqcm< -0.6

  8. Background reactions No-scale! (demonstration) Data p0 h w/r • 2,3 reactions are above • the 2s cut point. • 4 is much less than pion • production by ~2-3 • orders. 2s cut (~ 0.13) MC Missing mass square • gp  p p p • gp  p p p p • gp  ph • gp  pg

  9. Background subtraction Single pion and double pion productions generated by MC simulation are fitted to data by a template fit. Systematic uncertainty was estimated by doing a fits by Gaussians for p0 peaks with 2s and 1.5s cuts. Example of results of template fits

  10. Systematic error Determine differential cross section • Obtain yield by subtracting two-pion photoproduction. • Detector acceptance is obtained Monte Carlo • simulation based on GEANT3. • Photon normalization is obtained by tagging counter. • Target protons 1.0% (fluctuation of temp. and pressure) • Photon normalization 1.2%, Photon transmission 3% • AC over veto 1.6% • Background subtraction,< 5%

  11. Differential cross section, Eg LEPS data Existing data. Eur.Phys.J.A26(2005)399, PRL,94(2005)012503, PLB,48(1974)463, NPB60(1973)267… SAID -partial-wave analysis (fit of existing data) PRC66,055213(2002) MAID2005 - isobar model nucl-th/0603012 • Good agreement with existing data (SAID). • Not follow ~s2a(u)-2(Regge),~s7(counting rule) at Eg > 1.8 GeV.

  12. Differential cross section cosqcm LEPS data Existing data. SAID MAID2005 • Discrepancy with MAID becomes large at higher energy. • Change angular distribution drastically at Eg~1.8GeV, • Backward peaking, small bump structure 1.85<Eg<1.95GeV.

  13. Photon beam asymmetry S LEPS data Existing data. PLB544(2002)113 NPB104(1976)253… nNv- Nh PgScos(2f) = nNv + Nh SAID MAID2005 Positive sign: s < s Negative sign: s > s 2.1 GeV < W < 2.3 GeV • Data show a good agreement with SAID/MAID. • Strong angular dependence above 1.8GeV.

  14. Summary Differential cross sections at very backward angles have been measured and new data at Eg > 1.8 GeV. Photon beam asymmetries have been measured at the first time at backward angles Eg > 1.5 GeV. Angular dependence of both observables changes at Eg =1.8 GeV. A strong angular dependence is seen at Eg > 1.8 GeV. Energy dependence cannot be explain by nucleon Regge pole nor scaling. At low energy region, the data is explained with models including well-known-nucleon resonances, but the strong angular dependence at high energy cannot be explained. Can the data be explained by Born term only? Does u-channel contribution dominate? Quark-hadron duality? High spin states? Challenge to theorists. Measurement at side angles and higher photon energy, Homework to experimentalist.

  15. LEPS collaboration D.S. Ahn, J.K. Ahn, H. Akimune, Y. Asano, W.C. Chang, S. Date, H. Ejiri, H. Fujimura, M. Fujiwara, K. Hicks, K. Horie, T. Hotta, K. Imai, T. Ishikawa, T. Iwata, Y.Kato, H. Kawai, Z.Y. Kim, K. Kino, H. Kohri, N. Kumagai, Y.Maeda, S. Makino, T. Matsumura, N. Matsuoka, T. Mibe, M. Miyabe, Y. Miyachi, M. Morita, N. Muramatsu, T. Nakano, Y. Nakatsugawa, M. Niiyama, M. Nomachi, Y. Ohashi, T. Ooba, H. Ookuma, D. S. Oshuev, C. Rangacharyulu, A. Sakaguchi, T. Sasaki, T. Sawada, P. M. Shagin, Y. Shiino, H. Shimizu, S. Shimizu, Y. Sugaya, M. Sumihama H. Toyokawa, A. Wakai, C.W. Wang, S.C. Wang, K. Yonehara, T. Yorita, M. Yosoi and R.G.T. Zegers a Research Center for Nuclear Physics (RCNP), Ibaraki, Osaka 567-0047, Japan b Department of Physics, Pusan National University, Pusan 609-735, Korea c Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan d Japan Atomic Energy Research Institute, Mikazuki, Hyogo 679-5148, Japan e Institute of Physics, Academia Sinica, Taipei 11529, Taiwan f Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japan h School of physics, Seoul National University, Seoul, 151-747 Korea i Department of Physics, Ohio University, Athens, Ohio 45701, USA j Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan k Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan l Department of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japan m Department of Physics, Chiba University, Chiba, Chiba 263-8522, Japan n Wakayama Medical College, Wakayama, Wakayama 641-0012, Japan o Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan p Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan q Department of Physics, University of Saskatchewan, Saskatoon, S7N 5E2, Canada r Department of Applied Physics, Miyazaki University, Miyazaki 889-2192, Japan

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